The present invention relates to digital converters, or Resolver/Digital (R/D) converters, for digitizing the output of resolvers, and more particularly to an encoder output divider configured to divide an encoding output associated with an R/D converter.
A resolver is a type of rotary transformer typically having two stator windings and one rotor winding on an axis or shaft. A resolver is used for determining the position of the shaft, or more specifically the axis of the rotor with respect to a reference point within a space of one complete revolution of the shaft. One of ordinary skill will appreciate that any shaft position has a corresponding angular displacement within an angle space from 0° to 360°. The two stator windings are mechanically arranged such that their physical relation is shifted by a 90° angle. The physical spacing of the stator windings gives rise to a mathematical/electrical relation in that any signals induced in the stator windings from, for example, a rotor excitation signal, will be correspondingly shifted by 90°. Thus the sine and cosine function relations are conveniently assigned to the respective outputs of the stator windings to provide an electrical expression of the mechanical relation. It will be appreciated that the degree of accuracy of the 90° physical relation based on, for example, accurate placement of stator windings, will affect the degree to which the outputs are electrically shifted by 90° and thus accurately represent a sine and a cosine relation respectively. It is understood by those of ordinary skill in the art that the sine and cosine functions are mathematically characterized by a 90° shift therebetween.
The amplification of a signal obtained by coupling with the stator windings is a function of the position of the resolver rotor axis and the relative positions of the stator windings. Therefore, two types of output voltages, S3-S1, S4-S2, as may be measured between corresponding terminals of the two stator windings, are modulated according to a sine wave function and a cosine wave function corresponding to the axis angle of the resolver rotor relative to the stator windings. The output voltage waveforms can be expressed in the following formulae (1) and (2).S3-S1=A(sin ωt) (sin θ)  (1)S4-S2=A(sin ωt) (cos θ)  (2)where “θ” is the angle of the rotor axis, “ω” is the angular velocity corresponding to the rotor excitation frequency (f), and “A” is the rotor excitation amplification.
Proposals have been set forth for Resolver/Digital (R/D) converters in which, of the continuous output voltage signals S3-S1 and S4-S2, the signal with the smaller absolute value or magnitude is divided by the signal with the larger absolute value. Based on a resulting continuous signal associated with the quotient, the angle data is obtained. An example of such a proposed R/D converter may be seen, for example, in Japanese Unexamined Patent Application Publication S62-38302. Therein, tan θ and cot θ are calculated from output voltages S3-S4, S4-S2. At the same time, a digital code for A(sin ωt) is determined from the output voltage S3-S1, the digital code result such as an index, and the angle section of resolver rotor axis obtained from the digital code result or index. As noted, one of either tan θ or cot θ may be used as a digital code, and angle data associated with resolver rotor axis angle θ and corresponding to the digital code or index and stored in advance in a table, read from the table and output.
It will be appreciated that such rotor axis angle determination maybe useful for controlling rotational quantities such as rotational speed of rotating devices such as motors, generators, and other rotating machines. A block diagram showing an exemplary structure for a speed controlling system, such as a speed servo system or the like is shown in FIG. 3. Therein, a rotational device such as a motor may be characterized, for example, with regard to rotor speed by determining and tracking the rotor axis angle using the aforementioned R/D converter. In speed controlling system 300, R/D converter 33 converts a sine wave output and cosine wave output as, for example, analog signals from resolver 32 into digital signals, and outputs an encoding signal showing the rotor axis rotation angle of resolver 32 using, for example, methods known to those of ordinary skill in the art. Speed control system 300 thus carries out position control and rotational speed control of motor 31 in accordance with the encoding signal from R/D converter 33.
Conventional speed control systems such as speed control system 300 generate a speed signal representative of the speed of the rotor by calculating a difference signal or otherwise differentiating an encoding signal output of R/D converter 33 in differential circuit 34 where differences are preferably calculated for a given time constant. Problems arise however in that a typical speed signal generated in a conventional speed control system, such as speed control system 300, is known to be strongly correlated with angle error associated with the rotor axis angle of resolver 32 which in turn corresponds to the rotor axis angle or position of a rotor shaft associated with motor 31 or other device being measured and controlled. In conventional speed control system 300, angle error is manifested in resolver 32, for example, as one or more higher-order harmonic frequency components having a frequency characterized as an integral multiple of the rotational frequency or as a function of the angle of the resolver rotor axis.
Accordingly, an exemplary spectrum distribution associated with angle error components is concentrated in a relatively low frequency band such as near the rotational frequency and integral multiples thereof. In the case of R/D converter 33, which may be configured to generate an encoded output, angle error is manifested by adding noise components to the encoded output having a white noise-like spectrum distribution. Thus, unless a cutoff frequency of an exemplary integrator or low pass filter associated with the output of resolver 32 is lowered compared to a cutoff frequency of an exemplary integrator or low pass filter associated with the encoder, e.g. the output of R/D converter 33, torque or speed ripple cannot be easily reduced.
Further problems arise, however, in connection with lowering the cutoff frequency as described above. For example, when the cutoff frequency of an integrator or low pass filter associated with the output of resolver 32 is lowered compared to an encoder output such as the output of R/D converter 33, the open loop gain coefficient is lowered, thereby slowing the overall response time of speed control system 300 as would be appreciated by one of ordinary skill in the art. Thus, to obtain high precision positioning, a speed signal with low ripple must be generated without sacrificing the overall response time of the control system.
In accordance with some conventional solutions, a speed detector can be provided in addition to the rotor position detector. Further, the bit resolution of R/D converter 33 can be increased. Such solutions however, result in increasing the size and complexity of circuit and/or system structures, reducing circuit and/or system reliability, and increasing circuit and/or system cost.